Construction Of The Drilling Fluid Continuous Pressure Wave QPSK Signals And Its Transmission Characteristics Study In The Directional Wellbore

Measurement while drilling(MWD) is a kind of modern drilling assisted technique, through which down hole parameters can be measured and transferred simultaneously while drilling. On the basis of down hole information telemetry through drilling fluid channel, MWD is an effective method to transfer measured data while drilling. Enjoying the advantages of high information transferring rate and strong anti-interference, drilling fluid pressure quadrature phase shift keying (QPSK) modulation is the development trend of drilling fluid information remote measurement technique.According to working mechanism of rotary valve drilling fluid continuous wave signal generator the process of QPSK modulation were built. Drilling fluid pressure QPSK signals is PM signals generated by modulation on rotary valve speed by baseband signals. Based on signature analysis of baseband signals and Drilling fluid pressure QPSK signals, mathematical model of drilling fluid pressure QPSK signal was built, and, time domain and frequency characteristics of QPSK signal were analyzed. Research indicates that, since control object and control manner of baseband signal are different, drilling fluid pressure QPSK signals and the QPSK electrical signal of the conventional electronic communication system have some differences. While both of them have obvious frequency band, which can facilitates frequency band transmission of baseband data, and, frequency band moves with carrier frequency simultaneously, which can avoid the impact of interference signal on frequency band transmission data by changing carrier frequency. Under the same bandwidth, compared with binary data differential phase shift keying(BPSK), drilling fluid pressure QPSK modulation can double information transfer rate and bandwidth utilization efficiency, and modulated signal energy in regular band is just slightly smaller than that of BPSK signal. Through mathematical analysis of drilling fluid pressure QPSK signal, feasibility of QPSK modulation technique using in transferring down hole data at high speed was proved theoretically.Through hydrodynamic theory and equation of state of ideal gas, distribution of drilling fluid temperature and pressure along the directional well string was studied. According to acoustical theory, effect of drilling string in buildup section of directional well to signal transmission of drilling fluid pressure wave was studied. Research indicates that, radius of curvature is much bigger than string internal diameter, thus incident angle of pressure wave in buildup section is close to right angle, then total acoustic reflection is generated in inner wall of string during buildup section, and additional energy loss cannot be generated, so effect of buildup section to signal transmission is same to straight section. According to effect of temperature and pressure to drilling fluid gas-bearing ratio, mathematical model of pressure signal transfer function in directional well was obtained. Through numerical analysis method, effect of string internal diameter, carrier frequency, drilling fluid viscosity and gas fraction to drilling fluid pressure QPSK signal transmit from down hole to surface were studied.